1. Field of the Invention
This invention relates generally to the field of electron optics, and more particularly to electron detector optics for large substrate electron-beam testing systems.
2. Description of the Related Art
Electron beam systems employed for testing or inspection purposes typically generate a primary electron beam (or “probe”) which is focused onto the surface of a substrate by probe-forming optics. The signal detection process generally involves the collection of secondary electrons (SEs) and/or backscattered electrons (BSEs) which are emitted from the substrate surface as a result of the interaction of the primary electron beam with the substrate surface. In LCD substrate testing systems, the energy of the primary electron beam striking the substrate surface is generally in the range from 2 keV to 20 keV. SEs leaving the substrate surface have energies predominantly below 10 eV, while BSEs leaving the substrate surface predominantly have energies near that of the primary beam. The rate of generation of both SEs and BSEs from the substrate surface is proportional to the current in the primary electron beam, therefore variations in the primary beam current will lead to corresponding fluctuations in the detected SE and BSE signals. These fluctuations are undesirable since they cannot be distinguished from fluctuations arising from surface topography, elemental composition variations or changes in the surface voltage. Thus there is a need to generate a test signal which is not affected by fluctuations in the primary beam current.
Another requirement for electron beam testing of substrates, such as those used in the manufacture of LCD displays, is to be able to use the primary electron beam as a probe of the voltage on the surface of the substrate. This is possible if one of the signals is a monotonically-varying function of the surface voltage. The reason why it is necessary for the signal to be monotonic is to maintain a 1:1 mapping between the detected signal and the surface voltage—if the curve is not monotonic (i.e., the curve has the same signal level for N different surface voltage levels, where N>1), then there will be an N:1 mapping between possible values of the surface voltage and the detected signal and it will not be possible to determine the surface voltage unambiguously, as is familiar to those skilled in the art. A related requirement is for the signal to be a nearly-linear function of the voltage on the surface of the substrate—this requirement arises from the need to achieve the same signal-to-noise ratio, independent of the surface charging voltage. If the signal varies only a small amount for a large change in the surface voltage, then the signal-to-noise ratio will be low, while a large change in signal for a small change in the surface voltage will give a high signal-to-noise ratio. Generally, it is desirable to maintain approximately the same signal-to-noise ratio throughout the surface charging voltage range to obtain the same precision in the measured values of the surface voltage. Thus, there is a need to configure the design of the detector optics to make the SE detection efficiency a monotonically-varying and nearly-linear function of the surface voltage.
In many prior art electron beam systems, the secondary electron and backscattered detectors are positioned within the probe-forming optics, and a velocity filter using crossed electric-and-magnetic fields (commonly called a Wien filter, or an E×B filter) is used to separate three populations of electrons:                1) Secondary electrons coming up the column from the substrate are deflected by the Wien filter far off-axis into a first detector.        2) Backscattered electrons coming up the column from the substrate are deflected by the Wien filter slightly off-axis into a second detector.        3) Primary beam electrons passing down the column pass through the Wien filter with minimal deflection.        
There are several limitations to this approach:                1) Cost—a Wien filter requires complex machining of magnetic and electrode materials, as well as power supplies for the magnet coils.        2) Aberrations—the Wien filter always introduces some aberration (particularly chromatic) into the primary beam—this is a problem in low-voltage columns where the fractional energy spread (=ΔV/V0, where ΔV=the energy spread and V0=the column accelerating voltage) is larger.        3) Coupling between the SE and BSE signals—in many cases it is not possible to fully separate the SE and BSE signals, i.e., some of the SEs are collected by the BSE detector and some of the BSEs are collected by the SE detector.        
In other electron beam systems, the secondary and backscattered electron detectors are positioned below the probe-forming optics, and off to one side of the optical axis of the probe-forming optics. In these systems, it is necessary that the electric fields from the detectors do not substantially affect the primary beam. This requirement typically limits the detector collection efficiencies. With low detector collection efficiencies, the signal-to-noise ratio will be low.
Thus, there is a need for improved detector optics which provides high secondary electron and backscattered collection efficiencies combined with low cost, no aberrations induced in the primary electron beam, and fully-separated secondary and backscattered electron signals.